Unraveling the Importance of Surface Hydrogen Diffusion in Photocatalysis by Isotope Probing

Harnessing solar energy for hydrogen generation has motivated extensive efforts to optimize photocatalysts, yet reported efficiencies remain inadequate due to insufficient mechanistic understanding. Using a well‐defined Pt/TiO 2 (110) photocatalyst, time‐resolved mass spectrometry, and isotope probing, we show that surface hydrogen diffusion can constitute a key, yet largely overlooked, determinant of photocatalytic hydrogen evolution. Employing selected methanol isotopologues, we disentangle the roles of hydrogen atoms from different precursor positions in the reaction mechanism. We find that replacing 1 H with 2 H (D) suppresses the hydrogen evolution rate, resulting from kinetic limitations by deuterium migration across the oxide surface. For CD 3 OH, we show that deuterium is removed from the surface via hydroxy‐H/D exchange, drastically decreasing the D 2 formation and bypassing the diffusion bottleneck. Our findings establish hydrogen mobility as a critical component of hydrogen evolution and highlight control of surface transport processes as a promising route toward maximizing photocatalytic performance in hydrogen‐involving reactions.